This invention relates to an apparatus and method for first aligning and then positioning several tools or devices used in a welding operation for a lower portion of a damaged heat exchange tube extending in a bore of a tubesheet of a nuclear steam generator.
Nuclear steam generators are comprised of three principal parts, which are a primary side, a tubesheet in which a bundle of U-shaped tubes are mounted, and a secondary side. The tubesheet and U-shaped tubes hydraulically isolate the primary and the secondary sides, while thermally connecting them together so that heat from the radioactive water in the primary side is transferred to the non-radioactive water in the secondary side. This heat transfer is accomplished by the U-shaped tubes mounted in the tubesheet that extend throughout the secondary side of the steam generator. The inlet and outlet ends of these U-shaped tubes are mounted in the side of the tubesheet that faces the primary side of the generator. The primary side in turn includes a divider plate that hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends.
Hot, radioactive water heated by the nuclear reactor flows through the inlets in the tubesheet and circulates around the U-shaped tubes that extend within the secondary side of the steam generator. This hot, radioactive water transfers its heat through the walls of the U-shaped tubes to the non-radioactive water that surrounds the tubes in the secondary side of the generator, thereby converting the water to non-reactive steam. After the nuclear heated water circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, where it is ultimately circulated back to the nuclear reactor.
The U-shaped tubes are supported by several spaced-apart support plates and the inlet and outlet ends extend through the tubesheet. Over long periods of time, the heat exchanger tubes of such nuclear steam generators can suffer a number of different types of corrosion degradation, including intragranular stress corrosion cracking. Tests have shown that even though corrosion cracking occurs in the area of the tube supported in the support plates, that most of this intragranular stress corrosion cracking of the tubes occurs around the tubesheet region of the generator where the inlet and outlet ends of the U-shaped tubes extend through the bores in the tubesheet. These conditions can result in the tube becoming defective such that a leak occurs which not only creates an ineffective heat exchanger, but also creates a radioactive contamination problem. Since the fluid flowing in the tubes of the nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube must either be plugged or repaired so that the coolant does not leak from the tube.
Typically, the method used to repair a heat exchange tube in a nuclear steam generator is one in which a metal sleeve having an outside diameter slightly smaller than the inside diameter of the defective tube is inserted into the defective tube and attached to the defective tube to repair the defective or weakened area of the walls of the tube.
Examples of a sleeve loading mechanism is set forth in U.S. Pat. No, 4,711,526 issuing to Thomas E. Arzenti and William E. Pirl, and U.S. Pat. No. 4,829,648 issuing to Thomas E. Arzenti, William E. Pirl, and Annette M. Costlow. Both of these patents are assigned to the Westinghouse Electric Corporation and both are incorporated herein by reference.
Generally, laser welding is used to internally weld the sleeve to the tube in a heat exchanger since it is faster and produces a smaller heat affected zone compared to arc welding and brazing the joining metals, which were prior joining methods for welding the sleeve. An example of a device for laser welding of a sleeve within a tube is disclosed in U.S. Pat. No. 4,694,136 issuing on Sep. 15, 1987, to William H. Kasner et al., and assigned to Westinghouse Electric Corporation.
A further example of a device for laser welding of the inner surface of a tube is disclosed in U.S. Pat. No. 5,182,429, issued on Jan. 26, 1993, and assigned to Westinghouse Electric Corporation, which features a specific design for an elongated tubular welding housing which is inserted into the tube to be welded. In operation, the elongated tubular welder housing is introduced through the manway. The laser welding operation is followed first by a cleaning operation of the welded area by a water assisted hone brush which is rotated and pulled axially through the tube and then an inspection of the welded area by an inspection device.
Each of the several tools, such as the above discussed sleeve loading mechanism, the laser welding mechanism, the hone brush mechanism for cleaning the weld, and the inspection mechanism for inspecting the weld is individually manipulated within the radioactive, primary side of the nuclear steam generator by being independently and individually coupled to a robotic arm or a remotely operable service arm (ROSA), that has been invented and developed by personnel of the Westinghouse Electric Corporation. This robotic arm is located inside the nuclear generator by being attached to the tube sheet and extends through a manway in the bottom of the generator for access to the tooling.
Many of the tools discussed above are effectively used for inserting sleeves into and repairing a defective tube at the support plates for the U-shaped tubes or in the upper tubesheet region in the generator. If a sleeve is required at the bottom of the tubesheet, the normal practice is to perform a hard rolling expansion process which results in the joint being structurally sound, and which process is not normally performed on the joints between the sleeve and tube at the support plates and in the upper tubesheet region due to inaccessibility of these areas. However, in some instances, welding of the joint between the sleeve and the damaged tube at the bottom of the tubesheet may be desirable especially for gas, leak testing purposes. The several tools discussed hereinabove for performing a welding operation for the joints between the sleeve and the tube at the support plates and in the upper tubesheet region are generally not as effective for performing a welding operation for a joint in the bottom of the tubesheet since they are primarily designed to function while encapsulated within the tube and, preferably, the tools used for the joint in the bottom of the tubesheet should be designed to operate in air.
For a sleeving operation in a tube of a tubesheet for a nuclear steam generator, the initial cleaning operation of the entire tube is done with a water-hone brush which rotates and which is axially pulled through the tube, resulting in the joint between the sleeve and the bottom of the tubesheet also being cleaned. The several sleeves are inserted into the tube from the top down to the bottom of the tube, and hydraulically expanded at the same time. The joints are then welded from the top down. Normally, the joint at the bottom of the tubesheet cannot be properly welded since the present-day designs for the weld heads do not provide adequate constraint for the weld head in this bottom tubesheet area. The joints near the support plates and the upper part of the tubesheet normally require ultrasonic inspection due to the structural criteria, but the joint in the lower portion of the tubesheet only requires visual inspection.
There is a need for an apparatus used in the welding operation of a joint between the sleeve and the tube in the lower area of a tubesheet which constrains the several tools used in a welding operation.
There is a further need for an apparatus used in a welding operation of a lower joint in a tube in a tubesheet which is capable of making readily available the several necessary tools to accomplish the task. Ideally, such an apparatus should be easily and remotely manipulable within the radioactive environment of the primary side of the generator by means of a commercially available robotic arm. It would be desirable if the operator of the device combined the sequential steps of first positioning the end effector of the robotic arm, followed by welding, then cleaning the weld, and visually inspecting the weld by indexing and positioning the several mechanisms for doing these operations relative to a damaged tube being repaired so that the entire welding operation is substantially accelerated, and the maintenance operator's exposure to potentially harmful radiation is minimized.